Integrated circuits are chemically and physically integrated into a substrate, such as a silicon wafer, by patterning regions in the substrate, and by patterning layers on the substrate. These regions and layers can be conductive, for conductor and resistor fabrication. They can also be of different conductivity types, which is essential for transistor and diode fabrication.
Deposited conductors are an integral part of every integrated circuit, and provide the role of surface wiring for conducting current. Specifically, the deposited conductors are used to wire together the various components that are formed in the surface of the wafer. Such conductors are commonly known as "lines" or "runners". Conductors also provide other functions in integrated circuit structures, such as fuses and backside electrical contacts for the packaged die.
The conductive lines or runners are preferably formed of a highly electrically conductive material, such as metal. Another common conductive material for producing the surface wiring effect is polysilicon (hereafter poly). A concern in forming poly lines or runners is to protect the immediate area on either side of the runner from the next implant step. FIG. 1 diagrammatically illustrates in an enlarged and exaggerated sectional view a doped poly runner 12 which has been provided atop a thin SiO.sub.2 layer 11 on a doped silicon substrate 10. A photoresist mask layer 14 is provided atop the poly and is dimensioned to have a greater width than the desired finished width of the poly runner. As illustrated, this leaves a resist overhang on either side of poly runner 12 due to the greater width of the resist layer compared to that of poly runner 12.
This structure can be provided by isotropically dry etching the poly within a parallel plate reactor. A desired chemically reactive gas mixture is provided within such a reactor, and the mixture energized to a plasma state. The chemically reactive mixture is designed to be reactive with the poly, but not the photoresist or the silicon dioxide layer atop the substrate. The effect is to etch the poly runner to a narrower width beneath photoresist 14, as indicated by isotropic etching lines A. The term "isotropic etch" is an accepted term within the semiconductor industry which indicates an etching process which removes unmasked material in both a downward and sideward direction, as shown. Conversely, the term "anisotropic etch" defines an etching process which doesn't attack the masked material from the side, but merely etches in a downward direction. The isotropic etch of FIG. 1 is relatively simple since there is just one material being etched, namely the poly. The goal is to continue the etch until the poly runner is of the desired width. At the completion of the etch, an ion implantation would take place, the resist would then be removed, and the wafer would continue on through other process steps.
As the size of the integrated circuit shrinks, the speed that the current flows along the conductive runners becomes a critical issue. One way of increasing the speed of a poly runner is to deposit a more highly conductive metal silicide layer atop the poly. This invention concerns a particular metal silicide, namely WSi.sub.x. Such a layered poly lowers the total resistance of the runner.
The process of isotropically etching a WSi.sub.x /polysilicon sandwich is difficult because of the different etch characteristics of poly and tungsten (W). One of the usual outcomes of trying to etch both of these levels at the same time is presented in FIG. 2 where a tungsten layer 16 is etched at a faster rate than underlying poly layer 12. Dependent upon etch chemistry and etch processing parameters, the opposite problem is presented in FIG. 3 where the poly etch rate is much faster than the tungsten etch rate.
One of the ways that these problems have been overcome is by use of an anisotropic etch as illustrated in FIG. 4. Here, the resist is provided such that there would be no overhang of the finished width of the runner, with the anisotropic etch exemplified by lines B resulting in vertical sidewalls as illustrated. The resist would then be removed and the wafer passed through a spacer process to leave a small amount of oxide 18 on both sides of the conductive runner, as illustrated in FIG. 5. Oxide 18 acts as an implant block to provide the same function that a resist overhang does in an isotropic etch. However, this method involves several extra processing steps.
One object of this invention is to produce an isotropic etching effect in a WSi.sub.x /polysilicon sandwich structure beneath a layer of photoresist to provide a photoresist overhang of the finished width WSi.sub.x /polysilicon width runner.